Sheathed electric heating unit terminal end construction



June 30, 1959 J. L. SHROYER 2,392,989

SHEATHED ELECTRIC HEATING UNIT TERMINAL END CONSTRUCTION Filed June 27, 1955 3 Sheets-Sheet 1 June 30, 1959 J: SHROYER 2,892,989

SHEATHED ELECTRIC HEATING UNIT TERMINAL END CONSTRUCTION Filed June 27, 1955 s Sheets-Sheet 2 L 1N VEN TOR.

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June 30, 1959 J. L. SHROYER 2,892,989

SHEATHED ELECTRIC HEATING UNIT TERMINAL END CONSTRUCTION Filed June 27, 1955 3 Sheets-Sheet 5 102 I 55 I IN 1 l 4 INVENTOR. 69x i W 62 (76x05 Z. J'firayer fijX/ BY I? 24 90a 78 k United States Patent SHEATHED ELECTRIC HEATING UNIT TERMINAL END CONSTRUCTION Jacob L. Shroyer, Sepulveda, Calif., assign'or, by mesne assignments, to Ferro Corporation, Cleveland, Ohio, a corporation of Ohio Application June 27, 1955, Serial No. 518,055

11 Claims. (Cl. 338-239) This invention relates to tubular sheathed electric heating units and has reference more particularly to facilities for and a method of safeguarding such heating units at their terminal ends against entrance or accumulation of conductive substances which may cause current leakage or shorts.

Such heating units have one or more heating elements, usually in the form of elongated windings of high resistance wire, extending lengthwise within a metal tube, which is usually referred to as the sheath, and this tube or sheath has terminal conductors at one or both ends through which current is supplied to the resistance or resistances in the sheath.

Bare wires are commonly employed for the terminal conductors and have portions thereof extending into the end portion of the sheath and connected within the sheath to the resistances and also have outer end portions thereof projecting beyond the tube or sheath end for attachment of current supply conductors thereto, a low resistance wire usually being employed for these terminal conductors to avoid direct heating of the terminal end portions of the heating unit, and it is important that the exposed portions of these terminal Wires and the place of entry thereof into the tube or sheath end be protected against entrance or accumulation of any substances, to which the heating unit is exposed, which are capable of causing current leakage or shorts.

In such heating units it is customary to embed the resistances, and at least a substantial part of the length of the portions of the terminal wires which are within the sheath, in a mass of heat conductive electrically insulating refractory material, such as powdered magnesium oxide or the like, which is compacted to high density in the sheath and serves to electrically insulate the resistances and terminal wires from the sheath and each resistance and its terminal wires from other resistances and their terminal wires, and as presence of moisture in the compacted insulation impairs the electrical resistivity thereof, it is customary to dry the insulation thoroughly before or after, or both before and after, installation thereof in the sheath and to close the terminal end or ends of the sheath in a manner to exclude entrance of moisture into the sheath.

Moreover, it is customary to provide at the terminal end or ends of the heating unit a terminal block of ceramic or the like into which the terminal end or ends of the sheath project and the projecting portions of the terminal wires extend in channels or openings in this terminal block to terminal screws or the like with which the terminal block is provided for convenient attachment of current supply conductors thereto. A popular form of such terminal block is composed of halves which have individual channels or grooves in one or both inner opposed faces thereof to accommodate the projecting portions of the terminal wires in separated relation to one another.

While the tube end closing facilities and terminal block arrangements heretofore employed, as aforesaid, at the ICC terminal ends of such heating units have been sufficiently satisfactory for certain types of tubular sheathed heating units and conditions of use thereof, special conditions are encountered with certain types of heating units by reason of which previous terminal end facilities are not adequate to prevent occurrence of objectionable current leakage and shorts.

For example, range surface heating units and the like are operated at high wattages per unit of length and create intense heat and as the terminal ends thereof are subjected to proportionately high heat, the high temperature of the terminal ends of the heating unit and the expansion and contraction which occurs as a result of the wide range of temperature changes thereof, create conditions by reason of which previous tube end closures were inelfective to exclude moisture with certainty. The alternate heating and cooling of such high tem perature heating units creates a pronounced breathing elfect which is highly conducive to entrance of moisture, and when such heating units with previous end closing facilities were cooled or unused for long periods of time in an atmosphere of high humidity, atmospheric moisture would often penetrate and lower the electrical resistivity of the insulation within the tube sufliciently to cause shocks.

Moreover, range surface heating units have portions thereof at the terminal ends bent downwardly from the utensil supporting surface thereof so that the terminal ends are located below the range top where supply conductors may be connected thereto at an out of the way and concealed place, and grease and oils which are spilled in cooking readily run down these downturned ends and oftentimes reach the terminal end or ends of the heating unit, and because of the high temperature of the heating unit terminal ends and the high penetrativity and creepage tendency of the grease and oils at the terminal end temperature, such grease and oils would penetrate and accumulate sufiiciently within the tube and at the tube ends of previous range surface heating units to cause unit failure.

In such previous heating units a ceramic bushing having an individual opening therethrough for each terminal wire was usually inserted in the end of the tube and allowed to project beyond the tube end to increase the arcing distance between the terminal wires and the tube, and this ceramic bushing end closure provided capillary paths for grease and oils. to travel up the interior of the tube and eventually reach a place of sufficiently high heat to carbonize and cause electrical failure.

Moreover, the exposed outer end of the bushing provided a creep-age surface on which grease and oils would accumulate and cause electrical shorts between the terminal wires themselves or between the terminal wires and the tube.

Furthermore, previous terminal blocks did not have adequate provision for drainage of conductive liquids from the terminal wire containing openings or channels therein, and in the terminal blocks, for example, which were composed of halves, grease and oils would fre quently enter and accumulate between the halves and in the terminal wire containing channels and on surfaces between the channels and thereby cause shorts to occur between the bare portions of the terminal wires in the terminal block.

Other substances were similarly troublesome, and especially salt water spillage which sometimes occurred, as it is particularly conducive to shorts and readily caused shorts to occur when it accumulated in the terminal wire containing channels and on intervening surfaces therebetween within the terminal block or on intervening surfaces between the terminal wires or between the terminal wires and the tube at the end of the latter.

The principal objects of my invention are to safeguard such tubular sheathed electric heating units against current leakage and shorts which occurred as aforesaid in heating units as previously made; to provide a barrier in the terminal end or ends of the sheath which will effectively seal the sheath end; to eliminate the opportunity for accumulation of conductive substances on the heating unit end and in the terminal block which would be conducive to current leakage or shorts; to avoid any direct paths between the projecting terminals and also between the terminals and the sheath where conductive substances can accumulate and cause current leakage or shorts; to insure adequate drainage from places where such substances might otherwise collect; and to provide a method and simple and convenient facilities which are readily adaptable to tubular sheathed heating units to prevent current leakage or 81101118 and without any increase or material increase in cost, these and other objects being accomplished as pointed out more particularly hereinafter and as shown in the accompanying drawings, in which:

Fig; 1 is a top view of a tubular sheathed heating unit assembly of range surface heating type which is safeguarded against current leakage and shorts in accordance with the present invention;

Fig. 2 is an enlarged side view of a portion of the assembly of Fig. 1 looking at the left side of the terminal end of the heating unit of Fig. 1 and showing the downturned end of the heating unit and the mounting thereof;

Fig. 3 is a view similar to that of Fig. 2 but showing only the downturned end of the heating unit and mostly in section on the line 33 of Fig. 1;

Fig. 4 is a cross sectional view of the downturned heating unit end and mounting thereof taken on the line 4-4 of Fig. 2.

Fig. 5 is a cross sectional view similar to that of Fig. 4 but taken on the line 5-5 of Fig. 2;

Fig. 6 is a perspective view of the crushable bushing as prepared for use in the terminal end portion of the heating unit sheath to provide a barrier therein;

Fig. 7 is a top or inside view of the lower half of the terminal block of the heating unit taken on the line 7-7 of Fig. 2; i

Fig. 8 is a bottom or inside view of the upper half of the terminal block of the heating unit taken on the line 88 of Fig. 2;

Fig. 9 is an outer end view of the terminal block taken on the line 9--9 of Fig. 2;

Fig. 10 is a cross sectional view taken through the terminal block on the line 1010 of Fig. 2; and

Fig. 11 is a cross sectional view taken on the line 1111 of Fig. 2 and looking at the inner end of the terminal block.

For illustrative purposes, a range surface heating unit is shown herein which is similar to that disclosed in William R. Tuttle Patent No. 2,508,552 granted May 23, 1950, and comprises a tubular metal sheath, indicated as a whole by the reference numeral 15, which has a portion 16 thereof shaped in flat spiral form and supported in the usual manner in a horizontal position within the open top of a mounting shell or ring 17, this said shell or ring being provided with an annular flange 18 therearound by which it is supported in the opening of a range top 19 in the customary manner as shown fragmentarily at the upper right hand portion of Fig. 1.

The shell or ring 17 has a side wall portion 20 which extends downwardly through the range top opening and is provided at the bottom with a combination drain pan and reflector 21, which may be integral with the shell or ring 17 or a separate part mounted in the bottom thereof as is Well known in the art, and the spiral por tion 16 rests in the usual manner on radial arms '22 of a spider 23 which is supported by the shell or ring 17 at the proper elevation to hold the spiral 1.6 at the upper end of the shell or ring 17.

At its outer end the spiral portion 16 is provided with a straight substantially tangential tail-like extension 24 comprising an end portion of the sheath 15 which is bent downwardly at an acute angle to the plane of the spiral 16 and extends through an opening 25 in the side wall 20 of the mounting shell or ring 17 and is pivotally engaged in a bearing 26 on the side wall 20 at the outside thereof underneath the annular flange 18 and the range top 19 as shown particularly in Fig. 2.

This bearing 26 is arranged so that the spiral portion 16 rests solidly on the spider 23 and permits the spiral portion of the heating unit to be swung upwardly from its normal horizontal position on the spider to an upright position at one side of the shell or ring 17 to provide access to the underside of the spiral and to the interior of the shell or ring 17 and the pan 21 thereof for cleaning and also for cleaning the spider 23 which is usually unattached to the spiral portion 16 and removable from the shell or ring 17.

The tail portion 24 of the sheath 15 is round, whereas the spiral portion 16, in accordance with customary practice, is flattened at the top throughout its length from approximately the place indicated by the reference numeral 27 in Fig. 1 to provide a wide area of contact with the bottom of a utensil placed thereon, and this spiral portion 16 has two helical heating resistances 28 and 29 therein and extending lengthwise thereof at the top in side by side relation and a low resistance return conductor 30 extending lengthwise thereof at the bottom which are embedded in compacted heat conductive electrical insulating material 31, such as powdered magnesium oxide which is generally used for the purpose.

The sheath 15 is hermetically closed at the inner end of the spiral by an integral end closing wall portion 32, and within this closed end the inner ends of the two resistances 28 and 29 are connected to the inner end of the return conductor 30 in any convenient manner, as indicated at 33, with the place of such connection embedded in the insulation 31.

Preferably the return conductor 30 is a low resistance wire and is continued outwardly through the downturned end portion 24 of the sheath and has a portion 30 thereof projecting from the outer end of said sheath portion 24 for attachment of a current supply conductor thereto, Whereas the resistances 28 and 29 have low resistance wires 34 and 35 connected respectively thereto in the customary manner near the upper end of the downturned portion 24 and extending downwardly therein to the outer or lower end thereof where end portions 34 and 35 respectively project from said end of the sheath portion 24 for attachment of current supply conductors thereto.

In accordance with the present invention, the downturned portion 24 of the sheath 15 is not closed at its lower end, but instead has closing facilities inset therein so as to leave within that end a cavity 36 of substantial depth, for example approximately one half inch, at the inner end of which said cavity 36 the portion 24 has therein a molded close fitting ceramic bushing 37 which is provided with three openings therethrough which are in line respectively with the terminal wires 30, 34 and 35 and have said terminal wires extending respectively therethrough in a close fitting but slidable relation which permits the bushing to be slid along the terminals into position within the end of the portion 24 of the sheath 15.

The compacted insulation 31 extends well beyond the place where the terminal wires 34 and 35 are connected to the resistances 28 and 29 as is usual in heating units of this character, so that the places of such connection are embedded in the compacted insulation 31, and the said insulation 31 terminates at a considerable distance from the ceramic end bushing 37, as indicated at 38 in Fig. 3, leaving an intervening space between the insulation 31 and the bushing 37 which is filled with ceramic electrical insulating material 39 which is specially treated with a moisture and grease repellant oil and locally compressed to high density within the sheath portion 24 by contracting the portion of the sheath therearound, as at 40, throughout the major portion of the length of the insulation 39 to a sufiiciently reduced diameter to compact the material 39 to an impenetrable density and in effective sealing intimacy with the surrounding sheath portion 40 and with the terminal wires 30, 34 and which extend through the insulation 39 thereby providing a barrier to effectively exclude any substances which might detrimentally affect the heating unit or cause failure thereof.

This barrier 39 is prepared in the form of a rigid cylinder or bushing, as shown at 41 in Fig. 6, of porous crushable ceramic material of a diameter almost equal to the normal inside diameter of the end portion 24 of the sheath 15 so as to slide therein and with three openings 42 extending therethrough at proper places respectively to accommodate the terminal wires 30, 34 and 35 and this cylinder is impregnated with the moisture and grease repellent, which is preferably of silicone type and crushed to fine particles after it has been installed in position in the portion 24 of the sheath 15.

This cylinder or bushing 41 is made of steatite and formed by the extrusion molding process, and after it is formed it is air dried and then fired at a temperature of 2000 R, which is about l00O less than that used for making standard non-crushable bushings. The thus prepared cylinder or bushing is baked at a temperature of from 400 to 500 F. and then while it is still warm from the baking, it is immersed in the silicone liquid and allowed to remain therein until it becomes thoroughly saturated, after which it is baked at a temperature of 500 F. to cure the silicone content and is then ready for use.

After preparation, as aforesaid, the cylinder 41 is slid along the terminal wires 30, 34 and 35 into place within the portion 24 of the sheath against the end face 38 of the insulation 31 and the portion of the sheath 15 therearound is compressed as shown at 40, for example by simultaneously contracting multiple dies therearound, preferably with the bushing 37 or some other abutment jammed against the outer end of the cylinder 41 during the crushing and compacting operation.

The bearing 26 by which the heating unit is pivotally attached to the mounting ring 17 is composed of two separable parts, one of which is a vertical plate portion 43 of a bracket which is provided with upper and lower wing extensions 44 and 45 having flanges 46 and 47 respectively shaped to conform to the exterior of the side wall 20 of the mounting ring 17 below the flange 18 thereof and secured thereto by spot welding or the like to support the bracket on the mounting ring 17. The tail portion 24 of the heating unit extends along the side of the plate portion 43 and is held thereagainst 'by the other bearing part 48 which is in the form of a wide pipe strap arched as at 49 (see Fig. 4) to form a bearing seat for the tail portion 24, and this bearing part has flanges 50 and 51 respectively above and below the arched portion 49, which are secured to the 'vertical plate portion 43 by bolts 52.

The reduced diameter part 40 of the tail portion 24 is arranged so as to be located between the ends of the arched bearing portion 49 which has a correspondingly reduced intermediate portion 53 to correspond to the reduced diameter portion 40 of the sheath 15 and is stepped up at each end of the portion 53 to the normal diameter of the tail portion 24, as indicated at 54, so that the reduced diameter portion 40 of the tail portion 24 cooperates with the bearing 26 to prevent endwise movement of the tail portion 24 in the bearing.

A stop is desirable to limit the pivotal movement of the heating unit in the bearing 26 at the desired upraised position of the heating unit, and for this purpose an armate slot 55 is provided in the arched portion 49 of the bearing part 48 at the place of stepping from the intermediate portion 53 to one of the enlarged ends 54 and at this place the tail portion 24 of the sheath 15 is formed,

at the place of reduction thereof from the normal diameter thereof to the smaller diameter of the portion 40, with a small corrugation or nose-like extension 56 of said normal diameter portion which projects into the smaller diameter portion of the slot 55 and cooperates with the latter to provide a stop which, by engagement with the end 57 of the slot 55, limits the upward swinging of the heating unit at the desired place. The said slot 55 is, of course, of sufficient length to permit the winding 16 of the heating unit to be turned down to rest freely and solidly on the arm-s 22 of the spider 23 without restriction.

The inclined tail portion 24 extends beyond the hearing 26, as shown in Fig. 2, and at its lower end projects into the upper end of a correspondingly inclined terminal block which is indicated as a whole by the reference numeral 58 and is composed of two matching halves 59 and 60 which are secured together by two bolts 61 with their inner faces, which are indicated respectively at 62 and 63, engaged against one another, this terminal block being oriented on the end of the tail portion so that the half 59 thereof is at the bottom with the inner face 62 thereof facing upwardly when the heating unit is in the normal operating position with the winding 16 thereof resting on the arms 22 of the spider 23, at which time the half 60 is at the top with the inner face 63 thereof facing downwardly, the terminal block being shown in this position in Figs. 1, 2, 3, 9, 10 and 11.

The half sections 59 and 60 are made of insulating material and are usually molded ceramic parts because of the high temperature to which they are subjected, and each is provided with matching holes 64 for the bolts 61 and also other openings and channels, more or less matching to accommodate the projecting ends 30 34 and 35 of the terminal wires 30, 34 and 35 and terminal screws 65, 66 and 67 to which the projecting ends 30, 34 and 35 are connected.

At the end of the terminal block where the end of the portion 24 of the sheath or tube 15 is received therein each half section 59 and 60 is formed with a large substantially semi-cylindrical cavity portion therein, indicated respectively at 68 and 69, which conjointly form a large cylindrical cavity, indicated partly in dotted lines and partly in full lines at 70 in Fig. 9, in the assembled terminal block 58 and each cavity 68 and 69 is shouldered and enlarged at the outer end thereof to conjointly form at the outer end of the conjoint cavity 68, 69 a counter bore into which the terminal end of the portion 24 of the sheath extends, the said shoulder and counterbore enlargement of the lower half section 59 being indicated respectively at 72 and 73 and the shoulder and counterbore enlargement of the upper half section 60 being indicated respectively at 74 and 75.

The portion of the sheath end which projects into the conjoint counterbore 73, 75 has a grounding strap 77 secured thereto and this ground strap 77 has a substantially semi-circular portion 78 which conforms to and is secured to the bottom half of the end of the sheath with short and long end portions 79 and 80 projecting from opposite extremities of the portion 78 and all at one side of the plane of meeting inner face 62 and 63 of the half sections 59 and 60, as shown in Fig. 11, and the counterbore enlargement 73 of the lower half section 59 is appropriately enlarged and the half section 59 notched at opposite sides of the enlargement 73, as indicated at 81, to accommodate this grounding strap 77 which is provided with a screw 82 at the outer end of the portion 81 for connection of a ground wire thereto.

Thus this ground strap 77, which is secured in proper predetermined position of orientation on the end of the portion 24 of the sheath, serves by its location in the enlargement 73 and notches 81 of the lower half section 59, to assure orientation of the terminal block 58 on the sheath end with the half sections 59 and 60 at the bottom and top respectively thereof.

Near its end remote from the shouldered enlargement 75 thereof, and substantially mid'sxzi y of the width thereof, the half section is provided with a square recess 83 to receive the nut 84 of the terminal screw therein in a position substantially flush with the lower face 63 of said section 60, as shown in Fig. 3, and at places spaced apart from one another and from the square recess 83 said section 60 is provided in its inner face with two other square recesses 85 and 86 to receive respectively the nuts 87 and 88 of the terminal screws 66 and 67, the said recesses 85 and 86 being, however, of substantially greater depth than the recess 83, for reasons hereafter explained, and the half section 60 being provided with lumps 89 at the top with recesses 90 therein to accommodate the ends of the terminal screws 66 and 67, as shown in Fig. 10.

At places directly opposite the location of the recesses 83, 85 and 86 of the section 60, the other section 59 is provided with large openings 91, 92 and 93 respectively through which the respective screws 65, 66 and 67 may be inserted and access had thereto for operation thereof when the terminal block 58 is assembled.

The two terminal wires 34 and 35 are located in the upper half of the tail portion 24 of the sheath where their projecting portions 34 and 35 extend into the terminal block 58 above the plane of separation of the two halves, and to accommodate these projecting terminal wire portions substantially at the level or in the plane of their emergence from the end of the tail portion 24, the cavity 69 of the upper half 69 of the terminal block is provided with two large divergent branch channels 94 and 95 which lead respectively to the locations of the nut recesses 85 and 86.

The projecting portion 34 of the terminal wire 34 extends through the main channel 69 and outwardly therefrom through the branch channel 94 and has the outer end thereof looped around the terminal screw 66 and the projecting portion 35 of the terminal wire 35 extends through the main channel 69 and outwardly therefrom through the branch channel 95 and has the outer end thereof looped around the terminal screw 67, as shown in Fig. 8, for convenient connection of current supply conductors thereto.

To admit such current supply conductors into the in terior of the terminal block for connection to the loops of the projecting terminals 34 and 35 wide channels 96 and 97 are provided in the upper face of the lower terminal block section 59 which lead outward from the inner ends respectively of the screw access openings 92 and 93, and thus a current supply conductor may be inserted through the channel 96 and looped around the terminal screw 66 next to the loop of the terminal 34 with both loops located between the nut 87 and the head of the screw 66 and the two loops clamped in electrically conductive relation to one another by tightening the screw 66, and another current supply conductor may be inserted through the channel 97 and looped around the screw 67 and in like manner secured in electrically conductive relation to the loop of the terminal 35S by means of the screw 67.

The return conductor 30' is located in the lower half of the tail portion 24 of the sheath and the projecting terminal portion 31F thereof emerges from the end of the tail portion 24, at a level or in a plane below the bottom face of the upper half section 60 of the terminal block, and extends through the cavity 68 of the lower section 59 and therebeyond below the lower face 63 of the upper half section 60 and underneath a nose portion 98 of the latter which is located between the branch channels 94 and 95 thereof, to the terminal screw 65 and has the end thereof looped around said screw 65 as shown in Figs. 3 and 8.

To accommodate the terminal portion 30 beyond the cavity 68 of the terminal block section 59, said section 59 has a large cavity extension 99 leading from the cavity 68 thereof to the access opening 91 directly below the location of the center nut 84 of the screw 65 around which the end of the terminal portion 30 is looped as aforesaid, and to admit a supply conductor into the terminal block for connection to the looped end of the terminal portion 30 a wide channel 100 extends outwardly through the rear end of the lower section 59 from the access opening 91.

Thus a current supply conductor may be inserted through the channel and looped around the terminal screw 65 next to the loop of the terminal portion 30 and secured by the screw 65 in electrically conductive relation to one another in the same manner as the current supply conductors are connected, as above explained, to the looped ends of the terminal portions 34 and 35 In practice, the terminal block halves 59 and 60 are generally separated for connection of current supply conductors to the terminal portions 30 34 and 35 and, accordingly, the current supply conductors are not actually inserted through the channels 96, 97 and 100 but the latter merely accommodate and receive end portionsof the attached current supply conductors when the two halves 59 and 69 are assembled after the current supply conductors have been attached.

The center channel 100 in the outer end of the bottom section 59 is quite deep, for reasons hereafter explained, and the top section 60 is formed centrally at its rear end with a short depending lug. 101 which, when the sections 59 and 60 are secured together by the bolts 61, fits into the top of the channel 100 and keys the rear end of the section 60 to the rear end of the section 59 in proper superposed matching relation.

As shown particularly in Fig. 9, the channel 99, al though of less width than the cavity 68 from which it leads, is of the same depth as the cavity 68 and likewise the channel 100 is of the same depth so that there is no obstruction to drainage, and as the terminal block is inclined from front to rear at the same inclination as the tail portion 24 of the heating unit sheath, any fiowable substances received in the cavity 68 or channel 99 will drain readily to the access opening 91 and any such substances received in the channel 100 will drain from the rear end thereof. Preferably the channels 99 and 100 are of flat bottom form as shown in Fig. 9 to increase the capacity thereof.

In previous two piece terminal blocks of the type above described, the branch channels corresponding to the above mentioned channels 94 and 95 of the upper section 60, and likewise the channels of such previous terminal blocks corresponding to the above described channel 99 of the lower section 59 were of quite small cross sectional size and shallow and extended quite close to the counterbore at the front or upper end of the terminal block in which the end of the heating unit sheath was located, so that the bare portions of the terminal wires extending therein from the end of the tubular sheath to the terminal screws of the terminal block were confined to closely prescribed paths which were safely spaced from one another, and as the channel or channels of each half section of the terminal block were covered to a large extent by the opposing inside face of the other overlying or underlying half section of the terminal block, the bare terminal Wires in these previous channels were necessarily quite close to the channel walls and channel covering surface and even sometimes in contact therewith, and, moreover, because of their small size and location, such previous channels had little opportunity and in some cases no opportunity for drainage, and on account of the small channel size and also inadequate drainage, grease and oils and other substances would frequently accumulate in the channels and on the inside faces of the terminal block sections between the channels and provide conductive paths between the bare terminal wires and thereby cause electrical shorts to occur.

It is an important feature of the present invention that the channels 94, 95 and 99 are quite large in cross section, and indeed are so much larger than the terminal wires therein that there is practically no opportunity for close proximity of the terminal wires to the channel walls or contact therewith and, moreover, because of the large channel size and the narrow width of the barriers of the terminal block which separate the various channels and cavities from one another, neither half section 59 or 60 has any appreciable inner face area covering the channels of the other section and, accordingly, fiowable substances cannot accumulate in the channels 94 and 95 but instead drain into the cavity 68 and channel 99 from which they drain freely to the access opening 91, and, if they reach the channel 100 therebeyond, from the rear end of the channel 100 for the reasons explained above.

Moreover, in accordance with the present invention the portions of the terminal wires within the channels and cavities of the terminal block are not bare but instead each terminal wire portion 30 34 and 35 is individually enclosed, from its place of connection with its respective terminal screw 65, 66 and 67 to the ceramic bushing 37 within the end of the tail portion 24 of the sheath, in a continuous length of high temperature resistant silicone tubing 102 so that each terminal wire portion 30 34 and 35 is individually insulated throughout its length to its respective terminal screw and accordingly has no exposed surface to which grease, oil or other substance is accessible to cause current leakage or shorts to occur. This tubing 102 is preferably flexible so that the terminal wire portions enclosed thereby may be bent as required for location in the channels of the insulating block.

Moreover, the terminal covering tubes 102 are thin walled and because of the substantially greater cross sectional size of the channels and cavities through which they extend in the terminal block and the facilities for drainage provided therein, as hereinbefore explained, no opportunity is afforded for any appreciable accumulation of any conductive substances in the channels or cavities through which the insulating tubes 102 extend.

Thus, by reason of the larger size of the channels and cavities of the terminal block in which the terminal conductor portions 30 34 and 35 are located and the facilities for certain and adequate drainage thereof, and also by reason of the individual enclosing of each said terminal conductor portion in a separate tube of electrically insulating material, opportunity is precluded for current leakage or shorts to occur in the terminal block.

Moreover, by reason of the cavity 36 at the lower inclined end of the tail portion 24 of the sheath, the latter tends to shed grease, oil or other flowable substance from the sheath end and as the ceramic bushing 37 is inset within the end of the sheath, there are no exposed surfaces at the end of the sheath on which conductive substances can collect to provide conductive paths either between the terminal wires themselves or between the terminal wires and the sheath, and as the terminal wires are individually insulated at the place of entry into the sheath and throughout a substantial distance Within the end portion of the sheath as well as beyond the end of the sheath, the occurrence of current leakage or shorts at the end of the sheath is effectively precluded.

Because of the above mentioned shedding tendency of the sheath end and also because the ceramic bushing 37 is so far from the sheath end, and also because of the length of the insulating tubes 102 within the sheath end and the abutting thereof at their inner ends against the bushing 37, creepage of grease, oils or other fiowable substances into the sheath end to a place where such grease, oil or substances may cause current leakage or shorts is practically precluded, as well as capillary entrance of grease or oils past or through the bushing 37 to the interior of the sheath therebeyond.

Furthermore, the crushed silicone impregnated ceramic material 39 immediately beyond the bushing 37 has been found to be and provides an effective barrier to penetration of grease, oils and other fiowable substances and also completely prevents moisture being drawn into the tube or sheath during heating and cooling cycles or long periods of inactivity in an atmosphere of high humidity.

Thus the above described construction, by reason of the elimination of opportunity for current leakage or shorts to occur within the terminal block and at the end of the sheath and the elimination of creepage paths within the sheath end whereby grease, oils or other flowable substances can reach places to cause current leakage or shorts, and by reason of the sealing that is effected by the crushed silicone treated ceramic barrier, fully ac- .complishes the above stated purposes of the invention and the heating unit is effectively safeguarded at its terminal end against entrance or accumulation of conductive substances which may cause current leakage or shorts.

While I have shown and described my invention in a preferred form, and the invention is particularly adaptable to range surface heating units for the reason explained above, the invention or certain features thereof may be advantageously employed in connection with other electrical heating units and various changes and modifications may be made without departing from the principles of the invention, the scope of which is to be determined by the appended claims.

What I claim is:

l. A heating unit of the class described which comprises an electrical heating resistance extending lengthwise within and electrically insulated from a metal tube and wherein the said resistance has a terminal conductor leading therefrom through an end portion of the tube with a portion thereof projecting beyond the outer end of that end portion of the tube for attachment of a current supply conductor thereto, the said terminal conductor having a portion of the length thereof within said end portion of the tube embedded in and electrically insulated from the tube by a tube sealing mass of pulverized refractory material particles which contains a high temperature resistant moisture and grease repellent oil and is compressed to a reduced cross section of high density on the conductor within an intermediate portion of the length of said end portion of the tube which is circumferentially contracted on said compressed mass, and the said terminal conductor having the aforesaid portion thereof which projects beyond the outer end of the said end portion of the tube telescoped into a tube of high temperature resistant electrically insulating material which is spaced from the walls of and occupies a relatively small part of the cross section of a cavity in the interior of a rigid body of high temperature resistant electrically insulating material which is located at the outer end of the said end portion of the metal tube and has a drainage channel leading from said cavity to the exterior of said body.

27 A heating unit of the class described which comprises electrical heating resistance means extending lengthwise within and electrically insulated from a metal tube and wherein the said resistance means has a plurality of laterally spaced terminal conductors leading therefrom through an end portion of the tube and each of said conductors has a portion thereof projecting beyond the outer end of that end portion of the tube for attachment of a current supply conductor thereto, said terminal conductors having corresponding portions of the length thereof within said end portion of the tube embedded in and electrically insulated from one another and from the tube by a tube sealing mass of pulverized refractory material particles which contains a high temperature resistant moisture and grease repellent oil and is compressed to a reduced cross section of high density on the conductors within an intermediate portion of the length of said end portion of the tube which is circumferentially contracted on said compressed mass, the said terminal conductors having the portions thereof which project beyond the outer end of the said end portion of the tube individually telescoped respectively into separate tubes of high temperature resistant electrically insulating material which are divergently arranged in the interior of a rigid body of high temperature resistant electrically insulating material which is located at the outer end of the said end portion of the metal tube and has a cavity therein which contains the said divergently arranged tubes and wherein the said cavity has a drainage channel leading therefrom to the exterior of said body.

3. A heating unit of the class described which comprises electrical heating resistance means extending lengthwise within and electrically insulated from a metal tube and wherein the said resistance means has a plurality of laterally spaced terminal conductors leading therefrom through an end portion of the tube and each of said conductors has a portion thereof projecting beyond the outer end of that end portion of the tube for attachment of a current supply conductor thereto, said terminal conductors having corresponding portions of the length thereof within said end portion of the tube embedded in and electrically insulated from one another and from the tube by a mass of pulverized refractory material particles which contains a silicone compound and is compressed to a reduced cross section of high density on the conductors within said end portion of the tube with the tube circumferentially contracted on said mass, the said terminal conductors having the portions thereof which project beyond the outer end of said end portion of the tube individually telescoped respectively into separate tubes of electrical insulating material which are divergently arranged in the interior of a rigid body of electrical insulating material which is located at the outer end of said end portion of the metal tube, the said end por- 'tion of the metal tube having in the outer end thereof a recess in which the corresponding ends of all of said tubes of electrical insulating material are inserted and conjointly surrounded by the portion of the metal tube around said recess.

4. A heating unit of the class described which comprises electrical heating resistance means extending lengthwise within and electrically insulated from a metal tube and wherein the said resistance means has a plurality of laterally spaced terminal conductors leading therefrom through an end portion of the tube and each of said conductors has a portion thereof projecting beyond the outer end of that end portion of the tube for attachment of a current supply conductor thereto, said terminal conductors having corresponding portions of the length thereof within said end portion of the tube embedded in and electrically insulated from one another and from the tube by a mass of refractory material which contains a silicone compound and is compacted to high density in the tube with the said corresponding portions of the length of the terminal conductors embedded therein, the said terminal conductors having the portions thereof which project beyond the outer end of said end portion of the tube individually telescoped respectively into separate tubes of electrical insulating material which are divergently arranged in the interior of a rigid body of electrical insulating material which is located at the outer'end of said end portion of the tube, the said end portion of the metal tube having in the outer end thereof a recess in which the corresponding ends of all of said tubes of electrical insulating material are inserted and conjointly surrounded by the portion of the metal tube around said recess, and the said end portion of the metal tube having therein a ceramic bushing which separates said recess from the aforesaid compacted mass of refractory material and has openings therethrough through which said terminal conductors extend.

5. A heating unit of the class described which comprises electrical heating resistance means extending lengthwise within and electrically insulated from a metal tube and wherein the said resistance means has a plurality of laterally spaced terminal conductors leading therefrom through an end portion of the tube and each of said conductors has a portion thereof projecting beyond the outer end of said end portion of the tube for attachment of current supply conductors thereto, the said terminal conductors having successive corresponding portions of the length thereof from their inner ends to their outer ends extending respectively through a compacted mass of magnesium oxide and through a compacted mass of crushed ceramic material containing a silicone cornpound and through a bushing of solid ceramic material and through the inner ends of tubes of electrical insulating material which are located in a recess within the outer end of the end portion of the metal tube and through divergent portions of said tubes of electrical insulating material which are within the interior of a block of insulating material beyond the outer end of the end portion of the tube.

6. A heating unit of the class described comprising a metal tube which has a major portion of the length thereof bent in the form of a flat winding and has an end portion thereof bent downwardly from the plane of the flat winding, the flat winding portion of the tube having electrical heating resistance means therein and extending lengthwise thereof and electrically insulated therefrom and the downturned end portion of the tube having a plurality of laterally spaced terminal conductors therein which lead from said resistance means to the lower end of the downturned end portion and project beyond the lower end thereof for attachment of current supply conductors thereto, the said downturned end portion having an intermediate portion of its length compressed to a cross section substantially less than the portions of the tube between which said intermediate portion is located and containing a mass of electrical insulating material which is correspondingly compressed to a correspondingly less cross section, and a mounting in which the heating unit is pivoted, the said mounting having a bearing in which the said compressed intermediate portion of the downturned end portion of the tube is pivotally engaged and held against endwise movement by shoulders formed in the tube at the opposite ends of said compressed intermediate portion of the tube.

7. The method of sealing the terminal conductor containing tubular end portion of a metal tubular sheathed electric heating unit, which comprises preparing and impregnating with a moisture and grease repellent oil a preformed hardened rigid bushing of crushable ceramic material, then inserting the thus impregnated bushing in the tubular end portion of the heating unit with the terminal conductors extending therethrough, and then compressing the tubular end portion at the place where the said bushing is located therein and continuing such compressing until the bushing is crushed and compacted to a tube sealing density.

8. The method of sealing the terminal conductor containing tubular end portion of a metal tubular sheathed electric heating unit, which comprises preparing and impregnating with a silicone compound a preformed hardened rigid bushing of porous crushable material, then inserting the thus preformed bushing in the tubular end portion of the heating unit with the terminal conductors extending therethrough, and then compressing the tubular end portion at the place where the said bushing is located therein and continuing such compressing until the bushing is crushed and compacted to a tube sealing density.

9. A heating unit in accordance with claim 6 wherein one of the said shoulders of the tube has an offset projecting therefrom in the direction of the length of the tube and cooperable with an abutment of the mounting to limit pivotal movement of the tube in the bearing.

10. The method of making a heating unit with an electrical heating resistance extending lengthwise within and insulated from a metal tube which has an end portion projecting beyond one end of the resistance and containing a terminal conductor which is connected to the resistance and leads therefrom through said end portion of the tube and projects through and beyond an open end of said end portion for attachment of a current supply conductor thereto, which said method includes the steps of filling the resistance containing portion of the tube with a loose powder composed of particles of electrical insulating refractory material, then inserting in the said end portion of the tube at a place distant from the aforesaid open end thereof a preformed powder confining hardened rigid plug of ceramic material impregnated with a silicone compound which is already cured in the plug, and then locally reducing the cross section of that portion of the tube which encircles the plug and crushing, disintegrating and compacting the ceramic material of the plug therein to seal the passage through the end portion of the tube to the said refractory material therebeyond.

11. A heating unit of the class described comprising a metal tube which has the major portion of the length thereof bent in the form of a flat winding and has an end portion thereof bent downwardly from the plane of the flat winding, the fiat winding portion of the tube having electrical heating resistance means therein and extending lengthwise thereof and electrically insulated therefrom and the downturned end portion of the tube having a plurality of laterally spaced terminal conductors therein which lead from said resistance means through a locally compacted mass of sealing material to the lower end of the downturned end portion and project beyond the lower end thereof for attachment of current supply 14 conductors thereto, the portions of the terminal conductors which thus project beyond the lower end of the downturned end portion of the tube being individually telescoped respectively into separate tubes of high temperature resistant electrically insulating material which are divergently arranged in the interior of a rigid body of electrically insulating material which is located at the lower end of said downturned end portion of the metal tube, the said downturned end portion of the metal tube having in the lower end thereof beyond the locally compacted mass of sealing material a recess in which the corresponding ends of all of said tubes of electrically insulating materials are inserted and conjointly skirted by the portion of the metal tube around said recess and spaced apart from one another and from the metal tube.

References Cited in the file of this patent UNITED STATES PATENTS 2,272,282 Wiegand Feb. 10, 1942 2,357,150 Vogel Aug. 29, 1944 2,419,592 Richardson Apr. 29, 1947 2,506,554 Tuttle May 2, 1950 2,508,552 Tuttle May 23, 1950 2,610,281 Rutenber et a1. Sept. 9, 1952 2,617,001 Hasley Nov. 4, 1952 1,523,434 Lightfoot et a1 J an. 20, 1925 FOREIGN PATENTS 322,520 Great Britain Dec. 9, 1929 663,260 Germany July 7, 1938 816,282 Germany Oct. 8, 1951 

